Recently, the pressure sensitive cholesteric liquid crystal (ChLC) writing tablet, Improv Electronics® Boogie Board® LCD eWriter, has appeared on the market in which a pointed stylus or the finger can be used to write or trace an image on the surface of the tablet as described in U.S. Pat. No. 6,104,448 , which is incorporated herein by reference in its entirety. Improv Electronics® is a unit of Kent Displays, Inc. Such a stylus does not transfer any ink or other material to any surface. This tablet offers a considerable improvement over previous tablet technologies in that the image can be simply and instantly erased with the push of a button that applies a voltage pulse to electrodes in the tablet. In a ChLC tablet, the liquid crystal is commonly dispersed in a polymer network and sandwiched between two substrates that are spaced to a particular gap. The upper substrate is flexible and the bottom substrate is painted with a light absorbing (black or colored) background. The cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. When one presses on the top substrate with a pointed stylus, fingernail or other object, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from a substantially transparent to a brilliant reflective color at the location of the pressure. The reflective lighter color contrasts well to the dark background of the lower substrate. An image traced by the stylus or finger will remain on the tablet indefinitely until erased, typically consuming no power. Erasure is accomplished by applying a voltage pulse to substantially transparent conducting electrodes on the inner surface of the substrates that drive the ChLC from its color reflective state back to its substantially transparent state.
The above referenced technology and its applications are disclosed in more detail in U.S. Pat. No. 6,104,448, which is incorporated herein by reference in its entirety. Polymer dispersions of the cholesteric liquid crystal can be used to control the pressure sensitivity as described in U.S. Pat. No. 8,228,301, which is incorporated herein by reference in its entirety. Other modes of operation, including multiple color images, negative images and select erase, are described in the patent given immediately above as well as in U.S. Pat. No. 8,139,039, which is incorporated herein by reference in its entirety. Likewise, a variety of different versions of ChLC writing tablet have been developed where the background on the lower substrate is made semitransparent to allow for the use of templates, U.S. patent application Ser. No. 13/477,638, which is incorporated herein by reference in its entirety. Another version of a ChLC writing tablet, has patterned background or foreground markings, U.S. patent application Ser. No. 13/477,602, which is incorporated herein by reference in its entirety, to provide for a variety of different effects.
In order to provide image digitization, a touchscreen or dot pattern can be incorporated on the device such as disclosed in published U.S. Patent Application Pub. No. 2014-0043547 and U.S. patent application Ser. No. 14/069,901, which both are incorporated herein by reference in their entireties. Pressure from a stylus applied on the top substrate induces local shear driving the cholesteric liquid crystal from a dark state to a bright configuration in the stressed region. The drawing is simultaneously digitized and recorded. Therefore, an image is captured as a digital image in a storage unit to be later recalled on a computer or mobile device screen.
A disadvantage of this design is that eWriters produced in a standard fashion do not possess unique identifiers in the actual device, such as serial numbers, unique graphic designs or “watermark” (e.g., “digital watermark”) identifiers of interest to customers for logistics and security purposes. An approach is needed to provide an efficient and effective method in the production of the eWriter to create patterns on the eWriter that provide a visual effect similar to watermarks on paper. The disclosure herein describes a process of creating images with various degrees of visibility or shades of gray on a liquid crystal display (e.g., an eWriter display) that is subtle enough not to detract from a hand written image over it, yet distinctive enough to read.